Thousands of miles of buried natural gas pipes of varying size and formed from various materials are presently in service. All of these mains are in some state of progressive degradation. In most instances, the extent of such degradation is unknown, and hence, the serviceability of the mains is similarly unknown. This lack of information with the respect to the degree of degradation results in unforeseen gas pipe leaks and/or breaks, and necessitates the expending of substantial time and expense in locating these defects so that repairs and/or replacement can be made. Because of the need to detect conditions which might result in gas pipe breaks and/or leaks, an apparatus has been developed for inspecting gas pipes, and such apparatus is usually referred to as a pipe line "pig" or "mouse." For purposes of clarity, such inspection apparatuses are referred to hereinafter as scan assemblies. Pipe line scan assemblies typically include a housing with a plurality of sensors, such as ultrasonic transducers, mounted to the outer surface thereof in a predetermined configuration or array to contact the inner surface of the gas pipe.
As a scan assembly advances axially down a gas pipe, the ultrasonic transducer associated therewith produces interrogation pulses which pass through a coupling medium and then intercept the surfaces defining both the inner diameter and the outer diameter of the gas pipe and any imperfections or flaws within the wall of the gas pipe. The surfaces defining the inner diameter and the outer diameter of the gas pipe and any imperfections or flaws within the wall of the pipe, in turn, cause the individual return pulses to be transmitted back to the ultrasonic transducer. By knowing the speed of sound in the different mediums through which the interrogation pulse travels (i.e., the coupling medium between the ultrasonic transducer and the pipe wall and the pipe wall itself), the thickness of the pipe wall can be computed by timing the difference between the return pulse from the inside surface of the pipe wall and the return pulse from the imperfection, flaw or outside surface of the pipe wall. A more thorough discussion of these principles is found in U.S. patent application Ser. No. 08/222,621, filed on Apr. 5, 1994, and entitled "Scan Assembly Structure", the disclosure of which is incorporated herein by reference.
It is well known in the industry to utilize ultrasonic transducers as sensors for inspecting gas pipes in the manner described above. Ultrasonic transducers typically comprise a ceramic disk fitted with two electrodes, one on the top surface and one on the opposite or bottom surface of the ceramic disk. When a voltage pulse is placed across the ceramic disk, it momentarily deforms into a dome shape. Since the deformation process is very fast, the change in mechanical shape works on the air or any other medium in contact with the ceramic disk. This work is absorbed by the adjacent medium as a mechanical vibration or pulse, thus the named transducer. This is a description of the transmit mode of a transducer whereby electrical energy is transformed into mechanical energy referred to as an interrogating pulse.
As mechanical energy, the pulse created is sent through the test material to be reflected from a target of interest, for instance, a surface defining a pipe wall or an imperfection in the pipe wall. Since time is used to calculate the thickness (i.e., the distance traveled to the target and back to the transducer), the target location and the target shape, there must be means for timing the "time of flight" of the pulse. This is typically accomplished by range gating in which a counter begins counting at the moment the interrogating pulse is generated, and at some later point in time when the return pulse is expected to be received, the system begins looking for the return pulse signal that represents a reflective pulse from the surface defining a pipe wall or an imperfection. The remaining data is gated out because it is assumed that no viable return pulse could return outside of the window of time in which the system is looking for the return pulse.
In the receive mode of an ultrasonic transducer, the transducer receives small return pulses in the form of mechanical vibrations that result from the interrogating pulse reflecting off the target of interest. However, only a small portion of the mechanical vibrations placed into the test material as an interrogating pulse by the ultrasonic transducer are reflected back to the ceramic disk by the target. The return pulses are coupled from the test material to the ultrasonic transducer by the same physical coupling material that was used to connect the transmitted interrogating pulse from the ceramic disk to the test material. When the mechanical vibrations hit the ultrasonic transducer, they cause a minute deformation in the ceramic disk. This changes the electrical characteristics of the ceramic disk which is detected, amplified, and filtered to produce an indication of the timing and amplitude of the return pulse. These return pulse properties can be related to the position of the target and its size by knowing the speed of sound through the test material and the energy attenuation characteristics of the test material.
Presently, the received return signals are compared with a predefined detection threshold for determining what is actually a return pulse and what is merely noise. This method has proven problematic due to the dynamic range of valid return signals. For example, a return pulse from the back wall of a gas pipe may be very strong for several interrogating pulses and then be very weak for several more. In a cast iron pipe, this could be the result of surface irregularities that scatter the input energy resulting in a weaker return pulse signal. Nonetheless, the return pulse from the back wall is equally valid in each case, and thus, must be reported as simply a "back wall." If the detection thresholds are set too high, the weak return pulses are ignored and the system would incorrectly report a very thin pipe wall at that point based upon the fact that it did not "see" a back wall. In addition, the amount of noise in the received return signal may vary over time during a scan, which if not corrected, will result in a false signal incorrectly positioning the back wall of the gas pipe. There are also circumstances under which the threshold level should be calibrated to the condition and/or material comprising the gas pipe being tested.
In view of the foregoing, it would be desirable to develop a method and apparatus for discriminating between actual data signals and noise, whereby the accuracy and reliability of the inspection is improved.